An example of a background locking device is described with reference to FIGS. 9 and 10. FIG. 9 is a view for explaining an unlocked state of the background locking device. FIG. 10 is a view for explaining a locked state of the background locking device.
The locking device is attached to each of a moving-side member and a fixed-side member. In these figures, the moving-side member is, for example, a seat, a door, a trunk lid or an engine hood, which is provided to be rotatable relative to a floor of a vehicle. The fixed-side member is, for example, a vehicle body. The locking device is a device that is capable of preventing the relative movement between the moving-side member and the fixed-side member (e.g., see Patent Literature 1).
The locking device includes a base 1 and a striker 3. The base 1 is provided in one of the moving-side member and the fixed-side member, and the striker 3 is provided in the other one.
A shaft 2 is provided on the right portion of the base 1, and a hook 7 is rotatably provided on the shaft 2. A shaft 4 is rotatably provided on the left portion of the base 1, and a pawl 9 is provided on the shaft 4. A groove 1a is provided between the left portion and the right portion of the base 1.
The hook 7 is formed with a groove 7a into which the striker 3 is capable of entering. The hook 7 is formed with a concave portion 7b. The pawl 9 is formed with a convex portion 9a which is engageable with the concave portion 7b of the hook 7.
A spring 11 is caused to exert a force on the hook 7 and the pawl 9. One end of the spring 11 is locked to the hook 7 and the other end thereof is locked to the pawl 9. The spring 11 urges the hook 7 in an unlocked position direction (in the drawing, a direction of an arrow UL) and urges the pawl 9 in a direction (in the drawing, a direction of an arrow K) in which the convex portion 9a is engaged with the concave portion 7b of the hook 7.
A striker-contact member 13 is rotatably provided on the shaft 2 of the hook 7. One end of a spring 15 is locked to the base 1, and the other end thereof is locked to the striker-contact member 13. The spring 15 urges the striker-contact member 13 in the same direction as the unlock direction (in the drawing, the direction of the arrow UL) of the hook 7. The striker-contact member 13 is formed with a cam hole 13a. 
The pawl 9 is provided with a pin 17 which is fitted into the cam hole 13a of the striker-contact member 13.
As shown in FIG. 10, a stopper portion 13b is formed in the cam hole 13a of the striker-contact member 13, and the pin 17 of the pawl 9 is capable of abutting against the stopper portion 13b. 
The striker-contact member 13 is formed with a contact portion 13c, which is abutted against the striker 3 so as to sandwich the striker 3 in cooperation with a wall surface of the groove 7a of the hook 7 and a wall surface of the groove 1a of the base 1.
Next, an operation of the locking device configured as described above is described.
In the unlocked state shown in FIG. 9, the stopper portion 13b of the cam hole 13a of the striker-contact member 13 is abutted against the pin 17 of the pawl 9, and thus the striker-contact member 13 is prohibited from being further rotated in the direction of the arrow UL.
When a surface in the vicinity of an opening of the groove 7a of the hook 7 is pressed by the striker 3, the hook 7 is rotated in a locked position direction against an urging force of the spring 11. Then, the concave portion 7b of the hook 7 is engaged with the convex portion 9a of the pawl 9, so that the hook 7 is prohibited from being rotated in the unlock direction. In this way, the striker 3 becomes the locked state shown in FIG. 10, in which the striker 3 cannot be detached from the groove 7a of the hook 7.
When the convex portion 9a of the pawl 9 is engaged with the concave portion 7b of the hook 7, the pawl 9 is rotated in a direction of an arrow K. In this way, the abutment between the pin 17 of the pawl 9 and the stopper portion 13b of the cam hole 13a of the striker-contact member 13 is released. Further, the striker-contact member 13 is rotated in the direction of the arrow UL by an urging force of the spring 15. Then, the contact portion 13c of the striker-contact member 13 sandwiches the striker 3 in cooperation with the wall surface of the groove 7a of the hook 7 and the wall surface of the groove 1a of the base 1. In this way, the rattling between the striker 3 and the groove 7a of the hook 7 is eliminated.
On the contrary, in order to unlock the locked state shown in FIG. 10, the pawl 9 is rotated in a direction opposite to the direction of the arrow K against the urging force of the spring 11 and the spring 15. Then, the engagement between the convex portion 9a of the pawl 9 and the concave portion 7b of the hook 7 is released and the urging force of the spring 11 causes the hook 7 to be rotated to the unlocked position where the striker 3 is capable of entering the groove 7a. When a rotation operating force to the pawl 9 is released in this state, the convex portion 9a of the pawl 9 becomes the unlocked state shown in FIG. 9 by being abutted against the portion of the hook 7 other than the concave portion 7a. 
The pin 17 is also moved by the movement of the pawl 9, and the stopper portion 13b of the cam hole 13a of the striker-contact member 13 comes to push the pin 17.